In recent years, MOS sensors operable at low voltage have attracted attention in their application to portable equipment and the like. A conventional problem of MOS sensors that the resultant image quality is inferior compared to CCD devices is becoming improved.
To achieve both high-definition still-picture imaging and high-speed moving-picture imaging with one MOS sensor, a solid-state imaging device that performs thinning drive in which pixels in the MOS sensor are thinned in reading has been developed.
In a solid-state imaging device made of a MOS sensor, which can read only signals from pixels to which a selection pulse has been supplied, the thinning drive can be realized by supplying no selection pulse to pixels that are to be skipped in reading.
However, photoelectric conversion is performed also for skipped pixels. Therefore, if the thinning drive is realized by simply not selecting a pixel, a signal overflowing from a photoelectric conversion element of the skipped pixel may flow into an adjacent pixel, possibly causing generation of a false signal.
To prevent the generation of a false signal, a method is proposed in which charge in the photoelectric conversion element of a pixel to be skipped is released to a power supply terminal by putting a reset switch of the pixel to be skipped in the active state at all times (see Japanese Laid-Open Patent Publication No. 2000-350103, for example).
With the operation described above, it is possible to prevent such an occurrence that charge accumulated in the photoelectric conversion element of a pixel of which read is skipped during the thinning drive may be saturated and flow into an adjacent photoelectric conversion element causing generation of a false signal. In this way, a high-quality image hardly causing smearing, blooming, color mixing or the like can be obtained.
However, the conventional solid-state imaging device and drive method for such a device described above have the following problem.
In general, a solid-state imaging device has a plurality of n-type semiconductor layers in a p-well formed over the entire imaging region, in which each pn junction constitutes a diode. Contacts for grounding the p-well to the ground potential are not placed inside the imaging region to maximize the size of photodiodes, and the p-well is only grounded via contacts and the like in a region surrounding the imaging region. In this case, however, the potential of the p-well is not sufficiently fixed in the center of the imaging region. For this reason, when selection switch pulses and reset switch pulses are applied to signal lines, the potential of the p-well varies due to capacitance coupling between the signal lines and the p-well, and a fixed time must be secured until the potential of the p-well is stabilized. Thus, since it takes time to perform read and reset operation, a higher frame rate is unobtainable even though pixels are thinned for reading.
In view of the above, an object of the present invention is providing a solid-state imaging device having a high frame rate in which the potential of the p-well is prevented from varying during thinning drive and thus the read and reset operation is shortened.